Existing worm speed reducers, such as a speed reducing mechanism for a printer described in Patent Document 1, include a worm, which is fixed to an output shaft of a motor, and a worm wheel that meshes with the worm. In such a worm speed reducer, backlash is required. Patent Document 2 describes a technology for reducing the backlash of a worm speed reducer. According to the technology described in Patent Document 2, the speed of input rotation is reduced in the speed reducer by a worm gear fixed to an input shaft and a worm wheel fixed to an output shaft. The worm gear is hourglass-shaped and has a spiral ball groove formed therein, and the ball groove meshes with balls that are disposed in multiple recesses formed in the periphery of the worm wheel, whereby the rotational power of the worm gear is transmitted to the worm wheel.    [Patent Document 1] Japanese Unexamined Utility Model Registration Application Publication No. 5-16329    [Patent Document 2] Japanese Unexamined Patent Application Publication No. 5-302649